Clean-up usage of the old system UI flags in DisplayPolicy

This patch removed unnecessary code for the old insets mode and also
replaced the system UI flags, requested system UI flags, layout flags
with the new insets APIs.

This patch doesn't change any functionality of the system. Everything
should work as before.

Bug: 155413635
Test: go/wm-smoke
Test: atest DisplayPolicyInsetsTests
Test: atest DisplayPolicyLayoutTests
Test: atest DisplayPolicyTests
Change-Id: I0bf893dd85b3cd8282919864bbe52a94be47d6bb